Our preliminary results suggest that the chemokine receptor CCR9, by virtue of its-capacity to direct cell chemotaxis, and its ligand TECK through selective expression in the gastrointestinal epithelium, will prove of fundamental importance to mucosal immune and inflammatory responses. We hope to address a number of critical questions under this proposal: How is CCR9 regulated during B and T lymphocyte differentiation, and among lymphocytes infiltrating intestinal vs. other inflammatory sites? Is it dysregulated in intestinal inflammation --- e.g. in the inflammatory bowel diseases (]IBD)? Does the differential expression of CCR9 correlate with and help determine the localization or trafficking of lymphocytes to mucosal lymphoid tissues and lamina propria in small intestines and other GI sites? What role do CCR9 and TECK play in lymphocyte recruitment from the blood? Does CCR9 expression define the subset of memory T cells that Carries memory for intestinal recall antigens? Does it help target the humoral response in intestines as well? Flow cytometric and cytokine assays will characterize the phenotype and functional significance of CCR9+ lymphocyte subsets in man, and transwell assays of lymphocyte subset chemotaxis will be used to ask if CCR9 and TECK play a selective role in intestinal vs. systemic T cell migratory responses in mice as well. In vitro assays of T cell memory responses, and in vivo assays of immunity in the mouse model of rotavirus infection, will determine whether CCR9+ T cells comprise circulating memory for intestinal recall antigens. In situ hybridization will further define sites of TECK mRNA expression, and immunohistochemistry will be used to explore TECK distribution at the protein level in intestines and other tissues. In situ videomicroscopic analyses will be used to evaluate, in physiologic assays, the role of CCR9 and TECK in lymphocyte trafficking to normal and inflamed intestines. Finally, the expression of CCR9 and Of chemotactic responsiveness to TECK in the B cell lineage will be analyzed to define the role of CCR9 in B cell development and humoral immunity in the gut. The studies proposed will define critically the importance of CCR9 and its ligand TECK for homing of lymphocytes to the intestinal lamina propria, their role in intestinal immunity, and their potential as a therapeutic target in inflammatory bowel dieases.